1. Field of the Invention
The invention relates to DC-to-DC converters of the forward type. More particularly, the invention relates to forward power converters utilizing interleaved timing in a multiple winding dual-inductor buck switching configuration.
2. Description of the Prior Art
Converter topologies for DC-to-DC conversion commonly realized may be classified into the buck, boost and buck-boost switching regulators. Such converters may include a semiconductor switching element which when turned on and off couples energy from a DC source to an inductor in the converter. This energy is then transferred from the inductor to the DC output. Common DC-to-DC converter topologies include the buck (or forward) converter, the buck-boost (or flyback) converter, and the boost converter topologies.
In the conventional buck converter, a semiconductor switch is coupled between the DC supply input and the inductor, which in turn is connected to the DC output. A diode, normally reverse-biased, is coupled at a junction between the switch and the inductor to ground, polarized so that it is non-conductive when the semiconductor switch is closed. A capacitor is then connected between the output and circuit ground. During normal operation of the conventional buck converter, closing the semiconductor switch impresses the difference of the input voltage and output voltage across the inductor, causing the current in the inductor to increase and charging the output capacitor while also delivering current to any load connected to the output. When the semiconductor switch is turned off, the voltage at the junction between the switch and the inductor decreases until the diode is forward-biased. Current then flows through the diode and the inductive element with a decreasing amplitude, until the switch is closed and the cycle repeated.
An improved converter of the buck switching type is described in U.S. Pat. No. 4,713,742, "Dual-Inductor Buck Switching Converter", invented by David W. Parsley and assigned to the assignee of the present invention. In that invention a DC-to-DC power converter provided two parallel-connected inductors in series with a DC input power source. A commutation switch was connected in series with each of the inductors. The commutation switches were controlled so that they have a duty cycle that is greater than 50%. The duty cycle provides interleaved timing such that whenever one of the switches turns on or off, the other switch is engaged during the switching time. As a result of this construction, the sum of the currents that flow through the inductors provides an output current to the load with greatly reduced ripple and substantially constant output current. Further, the overlapping currents drawn from the power line result in harmonics of reduced amplitude and higher frequency content, thus permitting reduced size filter components to minimize input current harmonics. However, the aforesaid Parsley patent required a common connection between the input and output power supplies, therefore providing an output of only a single polarity.
In U.S. Pat. No. 4,618,919, "Topology for Miniature Power Supply with Low Vbltage and Low Ripple Requirements", invented by H.C. Martin, Jr., also assigned to the assignee of the present invention, a dual transformer switching topology provided isolation of the input and output power terminals. Further, a storage capacitor applied at the input of the power supply was switched sequentially in accordance with an interleaved timing to provide continuous filter support for the output voltage produced by the power supply. The capacitor performs the filtering function for the output voltages and also stores energy at high voltages as a result of the turns ratio of the power supply transformer, thereby permitting use of a capacitor with reduced physical size and capacity. The Martin patent was of the interleaved flyback converter type which, however, lacks inherent stability over a wide band of attenuation of input ripple.
The present invention is of an interleaved forward converter and as compared to conventional buck converters produces smaller ripple currents to be filtered by the filter capacitor, thus permitting a reduction in size of that element. Since the AC current components are reduced, smaller switching losses will be experienced. Ihe invention provides a wider band attenuation of input ripple than an interleaved flyback regulator because of the greater inherent stability of the buck derived topology over buck-boost derived topologies. Two buck switching power stages are operated 180.degree. out-of-phase, with the input nodes of the inductors coupled to a common filter capacitor. On each inductor core, a secondary coil is wound which is switched to a load at the time the primary winding is shunted across the input filter capacitor. In this way, dual-inductor buck power operation is provided while maintaining input-output isolation. The circuit provides DC-to-DC power conversion with isolation between input and output at high efficiency and high power density.